Rare earth elements are defined as a set of seventeen chemical elements from the periodic table, the fifteen lanthanides plus yttrium and scandium. Light rare earths are defined as the first five lanthanides (lanthanum, cerium, praseodymium, neodymium and promethium—the last one being unstable in nature) plus yttrium and scandium. Medium rare earths, or SEG, are samarium, europium and gadolinium, leaving the other seven (terbium, dysprosium, holmium, erbium, thulium, ytterbium and lutetium) as heavy rare earths. Rare earths are moderately available in the earth crust. Some light rare earths are even more abundant than nickel, cobalt or lead.
There are many rare earth-bearing minerals, from primary or secondary formation. Usually, rare earths are explored as by or co-product of another operation. Commercial operations of rare earths as the main product are usually from monazite or bastnasite deposits, being those two the main minerals for the industry.
Rare earths extraction from these minerals is widely known by the industry and is considered state-of-the-art processing. After they are submitted to a mineral concentration stage, known by those skilled in the art, a pure rare earth concentrate is submitted to acid or caustic attacks. These processes work well for monazite or bastnasite concentrates, where the rare earth minerals have a good degree of separation from other impurities, like iron or aluminum. These processes, however, cannot be applied to a deposit where poor mineral concentration is achieved.
Caustic cracking, one of the available processes for attacking monazite concentrates, uses caustic soda to attack the rare earth phosphates, producing rare earth hydroxides and soluble TSP (tri-sodium phosphate), a valuable by-product. This operation usually occurs at 120-150° C. using agitated tanks. After solid-liquid separation, the rare earth hydroxides are leached in HCl and sent to further processing, usually solvent extraction. TSP is removed from the solution by crystallization methods. This process cannot cope with high silicon, aluminum and/or iron concentrates. Aluminum and silicon can be leached by caustic soda consuming reagents and increasing solution viscosity. Iron and these elements can also produce colloids. These effects make operations very difficult as solid-liquid separation becomes a challenge and caustic soda consumption increases considerably.
The other state of art process for monazite, attack with hot and concentrated sulfuric acid, called sulfation, mixes a high amount of concentrated (96-98%) sulfuric acid (several times over stoichiometric amounts) with the rare earth concentrate, heating the mixture around 200-250° C. to increase kinetics. Rare earth sulfates are formed and later dissolved by adding water. The rare earth sulfates are removed from solution as sodium double sulfates or as oxalates. These are attacked with caustic soda, forming hydroxides that are leached with HCl. The HCl solution is usually taken to solvent extraction for proper elements separation. High levels of impurities like iron, aluminum or alkali and earth-alkali elements will increase acid consumption and make solution purification more difficult and costly.
Processes for bastnasite extraction are very similar to monazite sulfation. The concentrate can be attacked with hot sulphuric acid, as the Bayan-Obo process, or with calcination followed by hydrochloric acid leaching, as the Mountain Pass process. The same issues seen for high impurity monazite concentrates using the sulfation method would be seen here as well.
For those reasons, state-of-the-art processes cannot be applied to deposits where poor mineral concentration is obtained. High levels of acid (or caustic) consuming elements make it very difficult to use one of the widely available processes. There have been several developments to try and go around this issue with low grade, high impurity rare earth ores.
One such publication, FR2826667, by Renou & Tognet, is a patent application that teaches that heating a mixture of fine rare earth ore (particle size of 100 μm or smaller), containing high levels of iron, and sulfuric acid, at a ratio of 1 and 2, to a temperature higher than 780° C. but lower than 820° C. for 1 to 3 hours is sufficient to obtain rare earth sulfates that will solubilize without any iron in solution at a later stage where water is added. This invention has several drawbacks, as a high acid consumption (up to 2 times acid to ore ratio) and the need of very high temperatures. The mechanism involved is to convert all species in the ore into sulfates (including the impurities) and decomposing them at high temperatures into insoluble oxides, releasing SOx. This invention takes care of reducing the amount of impurities added but still needs a high amount of sulfuric acid to be added and high temperatures to decompose some of the sulfates formed. Rare earth sulfates may also decompose to some extent, reducing overall extraction rate. It may produce a pure rare earth solution but does not take care of the high acid consumption.
Another document, by Huang et al. (WO 2009/021389), teaches that heating up the acid and ore mixture, at an acid ratio of 1 to 2 times the ore mass, between 231 and 600° C. is enough to obtain a high rare earth extraction with low impurities in solution. This invention applies a much lower temperature than the previous one, but with similar results. This invention increases temperature to dehydrate sulfates of some elements, like iron, but other sulfates, like rare earths, are not yet dehydrated. These dehydrated compounds are not readily soluble, so by controlling conditions during the dissolution stage the inventors can reduce the amount of impurities that are put in solution. As the previous invention, this one does not reduce the amount of sulfuric acid needed, finding only a way of not dissolving the already formed iron sulfate.
The invention presented in this document differs from state of art processing and these other two inventions. The present document brings a process that indirectly attacks rare earth minerals using aluminum and iron sulfates that are formed by adding small quantities of sulfuric acid, enough to attack the said rare earth minerals. This process produces stable species of iron and aluminum, leaving rare earths as soluble species. It is, therefore, an indirect leaching process that attacks rare earth minerals not with sulfuric acid, but with iron and aluminum sulfates. FIG. 1 shows that neither of the discussed inventions are able to obtain similar results, as either iron is not converted into phosphates at low temperatures or rare earth sulfates start to decompose, together with impurities, at higher temperatures.